While early events in the pathogenesis of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) have been defined, little is known about mechanisms mediating resolution. To begin to search for potential determinants of resolution, we exposed wild type (WT) mice to intratracheal lipopolysaccharide (i.t. LPS) and assessed the response at intervals out to day 10, a time when injury had resolved. Bronchoalveolar (BAL) and lung mediators increased after i.t. LPS, among them BAL nitrites/nitrates (NOx). Consistent with the increase in NOx, we found that inducible nitric oxide synthase (iNOS) protein expression was significantly upregulated in the lung and peaked at day 4 after injury. Early lung injury was attenuated in iNOS-/- mice after i.t. LPS, however recovery by day 10 was markedly impaired in comparison to WT mice. iNOS-/- mice had increased mortality as well as persistently elevated BAL protein, albumin, cells, neutrophils and pro- inflammatory cytokines. Adoptive transfer of WT iNOS+/+ bone marrow-derived monocytes or direct adenoviral gene delivery of iNOS into injured iNOS-/- mice (given 1 and 2 days after i.t. LPS respectively) restored resolution of ALI in a pattern similar to WT;in contrast, transfer of iNOS-/- monocytes or delivery of sham adenovirus did not achieve resolution. To begin to understand how iNOS contributed to resolution of ALI, we performed multicolor flow cytometry of alveolar cells at intervals after i.t. LPS. We observed markedly decreased numbers of Regulatory T cells (Tregs) and a sustained expression of macrophage/monocyte co- signaling molecules (e.g. CD86 and CD40) in iNOS-/- mice compared to WT mice out to day 7 after injury. Transfer of supplemental WT Tregs or antibody-mediated blockade of CD86 in injured iNOS-/- mice remarkably restored resolution of lung injury. We hypothesize that macrophage-derived iNOS plays a pivotal role in mediating resolution of ALI by modulating immune responses in the lung, thus facilitating clearance of alveolar inflammation and promoting lung repair. To move these findings towards consideration in a clinical context, we propose the following specific aims: 1. To identify mechanisms by which iNOS modulates macrophage innate immune responses in the lung after injury. Using in vivo and in vitro approaches, we will determine the role of iNOS in modifying macrophage/monocyte innate immune responses at different stages of the injury resolution response. iNOS- mediated effects on macrophage co-signaling molecules and its relative contribution in resolution of ALI will be examined by using antibody blockade and creation of bone-marrow chimeras. We will compare the effects of exogenous vs. endogenous NO in modulating macrophage immune responses after injury. 2. To determine the role of iNOS in modulating alveolar macrophage-Regulatory T cell interactions to promote resolution of ALI. In vivo and in vitro studies will be used to evaluate the role of macrophage- derived iNOS in modulating Treg lymphocyte phenotype and function. The potential effects of Treg-derived iNOS will be also be investigated. 3. To determine if manipulation of iNOS can improve outcomes in different models of ALI. We will use direct adenoviral gene delivery of iNOS or NO donors in vivo, both to restore resolution of ALI in iNOS-/- mice after i.t. LPS and direct live gram negative bacteria model of ALI. Targets identified in SA 1 and 2 (e.g. Co- signaling molecules) will be manipulated in an attempt to accelerate resolution in 'severely'injured WT animals. Definition of mechanisms responsible for resolution of ALI may provide novel targets for therapy in an important clinical condition which kills more than 75,000 people annually, and for which therapy at present remains supportive. We believe targeting iNOS expression, particularly at later stages after onset of ALI, may prove useful as therapy to accelerate resolution in patients with ALI. PUBLIC HEALTH RELEVANCE: Definition of mechanisms responsible for resolution of Acute Lung Injury (ALI) may provide novel targets for therapy in an important clinical condition which kills more than 75,000 people annually, and for which therapy at present remains supportive. We believe targeting iNOS expression, particularly at later stages after onset of ALI, may prove useful as therapy to accelerate resolution in patients with ALI.